An ice dam is a hump of ice that forms at the edge of a roof under certain winter conditions. It can damage both your roof and the inside of your home. It will put gutters and downspouts at risk, too.

HOW DO ICE DAMS FORM?
Ice dams form when the roof over the attic gets warm enough to melt the underside of the layer of snow on the roof. The water trickles down between the layer of snow and the shingles until it reaches the eave, which stays cold because it extends beyond the side of the house. At the edge of the roof, the water freezes, gradually growing into a mound.
The flatter the pitch of the roof, the easier it is for an ice dam to get a grip. Gutters at the eaves can also trap snow and ice. If snow and ice builds up high enough in the gutter, it can provide a foundation for an ice dam.

WHAT DAMAGE DO ICE DAMS CAUSE?
Snow and ice melt can back up behind the dam and seeps underneath the shingles. Eventually, it will drip into the insulation and down into the ceilings and exterior walls beneath the eave, ruining sheetrock and paint, and forming black mold. If the ice dam breaks free, it can pull shingles and gutters off with it, and it will damage anything it falls on: shrubs, windowsills, cars, pets, and people. If the roof sheathing remains wet, it may rot.
(More on ice dams at http://www.IceDamCompany.com.)